This invention relates to a new process for the production of diisocyanates having a bis(isocyanatophenoxy)alkane structure, to new diisocyanates obtainable by this process and to their use as a synthesis component in the production of polyurethane plastics by the isocyanate polyaddition process.
DE-OS No. 3,442,689 describes certain bis(4-isocyanatophenoxy)alkanes which are comparable to 1,5-diisocyanatonaphthalene in their suitability for the production of high-quality polyurethane elastomers. Only those diisocyanates which contain unbranched alkylene radicals between the two ether oxygen atoms are specifically described. However, these known diisocyanates have certain disadvantages both in regard to their properties and in regard to their producibility, i.e., in regard to the producibility of the intermediates on which they are based. Their melting points (88.degree.-98.degree. C.) are relatively high and their solubility in apolar organic solvents is unsatisfactory. As a result, they are often difficult and comparatively expensive to process in the melt and in solution. The production of the dinitro compounds on which they are based by conventional methods is also uneconomical. The condensation of alkali 4-nitrophenolate with alkane dihalides only gives acceptable yields where expensive alkane dibromides are used. In addition, part of these expensive dibromides is irreversibly consumed in a secondary reaction with elimination of hydrogen bromide. This secondary reaction can become the main reaction where branched alkane dihalides are used. Sterically hindered alkane dihalides such as 1,3-dihalogen-2,2-dialkylpropanes cannot be reacted at all to form the corresponding bis(nitrophenoxy)alkanes by these processes.
Although DE-OS No. 3,442,689 mentions the reaction of 4-nitrochlorobenzene with alkane diols or with (4-nitrophenoxy)alkanols in the presence of bases as another method for producing the dinitro compounds, there is no reference whatever to the type of base to be used or to any solvents which may be present. The obvious choice of an aqueous sodium hydroxide as base leads to unsatisfactory yields of the corresponding dinitro compounds, even in cases where inert solvents, such as chlorobenzene, are used. In addition, a procedure such as this is totally unsuitable for the production of dinitro compounds containing a branched alkane radical. Use of alkali metals as "base" does not produce the desired result either, because secondary reactions occur to a considerable extent even in cases where monohydric alcohols are used (c.f. The Chemistry of the Ether Linkage, Ed. S. Patai, Interscience Publishers, 1967, p. 450; Houben Weyl, Vol. VI/3, p. 77).
The phase-transfer-catalyzed reaction of nitrochlorobenzenes with organic hydroxy compounds in aqueous/organic two-phase systems described in DE-OS No. 2,634,419 also leads to only traces of the desired bis(nitrophenoxy)alkanes where 2- or 4-nitrochlorobenzene is used. Instead, the main products obtained are dichloroazoxybenzenes and the corresponding nitrophenoxyalkanols.